Treatment of Retroperitoneal Fibrosis with Rituximab, Cyclophosphamide and Dexamethasone, Followed by Rituximab and Dexamethasone Maintenance, Achieved Disappearance of Pathological PET Accumulation of FDG and Regression of Fibrotic Masses after 4 Months of Therapy and the Patient is Still in Complete Remission after 3 Years. A Case Report and Iteraure Review.